Just to add to A of N's response, the grade you paid for will be the 'Minimum' you receive & if your lucky you may be allocated a higher grade. Good Luck.
Because a guarenteed cabin is exactly that. You pick a certain grade and you will be allocated a cabin on arrival (sometimes before). With a guarentee it works out cheaper, but you have to be happy to take what you are given - so you may have an obstructed view, be next to the elevators, be no where near the elevators, be in a noisy area.....but of course your T/A would have explained all of this to you.